Personally, i would come off and wait until it is fully healed to start back on cycle.

You should have minimal atrophy as long as you get back into the gym after the cast is off and take it easy. I wouldn't train just one side of your body unless it's extremely light. Just get some rest and hit your lower body hard, once your cast is off and you are clear to workout, get back into your routine slowly.

Wow...the same thing happened to a friend of mine back in November..sucks dude, sorry to hear that. I told him to see it as a blessing in disguise...sometimes we put so much attention into our bodes other aspects can begin to lack. Find some good books...take this time to improve yourself through other means. You're better off just doing what overload said.